The enthalpy of formation of magnesium sulfate ion pairs

The enthalpies of dilution of lithium, sodium, potassium, and magnesium chloride and sulfate solutions in the range 0.1 to 1.0 m have been measured at 30°C with a microcalorimeter. The relative apparent enthalpies φ_L of these solutions have been determined with the aid of an extended form of the Debye-Hückel limiting law $$phi _L = S_H I^{1/2} [1/(1 + I^{1/2} ) - (sigma /3)] + {rm B}{rm I} + CI^{3/2} $$ whereS _H is the limiting-law slope, σ = 3/I^3/2 × [(1+I^1/2 ? 1/(1+I^1/2 ? 2 ln (2+I^1/2)], andB andC are empirical constants. This equation fits the experimental results to within a standard deviation of 2 cal-mole^?1 for all the salts. The measured φ_L for the MgSO₄ solutions were compared to those calculated using the additivity principle, φ_L(MgCl₂) + φ_L(Na₂SO₄) ? 2φ_L(NaCl), and the extended Debye-Hückel equation. The results of this comparison have been used to calculate the ΔH _A ^o for the formation of MgSO ₄ ⁰ . A value of ΔH _A ^o = 1.15 to 1.36 was obtained, depending upon the φ_L estimates for the free ions Mg²⁺ and SO ₄ ^2? . The results are briefly discussed and compared to the results obtained by other workers.